Ascorbic acid (vitamin C) and various derivatives thereof are known to be compounds that exhibit effectiveness and efficacy in terms of skin whitening effects, antioxidative effects and collagen synthesis promotion effects and the like, and are therefore frequently added to pharmaceutical preparations, cosmetic materials and animal feed and the like.
Among ascorbic acid derivatives, ascorbic acid-2-phosphate derivatives in which the hydroxyl group at position 2 of ascorbic acid is converted to a phosphate ester are widely used due to their resistance to oxidation in air. Examples of known ascorbic acid-2-phosphate derivatives include ascorbic acid-2-phosphate and ascorbic acid-2-phosphate-6-higher fatty acid esters in which the hydroxyl group at position 6 of ascorbic acid-2-phosphate is esterified with a higher fatty acid such as palmitic acid. These ascorbic acid-2-phosphate derivatives are usually used in the form of salts. Typical examples of these salts include the sodium salt and magnesium salt of ascorbic acid-2-phosphate, and the sodium salts of ascorbic acid-2-phosphate-6-higher fatty acid esters.
Among the above derivatives, salts of ascorbic acid-2-phosphate-6-higher fatty acid esters, and particularly sodium ascorbic acid-2-phosphate-6-palmitate (hereinafter abbreviated as APPS), show considerable promise for application to skin preparations for external use such as cosmetic materials, because they are amphiphilic and therefore exhibit excellent compatibility with living organisms and their migration into biological tissues such as skin is rapid.
The advantages of APPS over other ascorbic acid derivatives are disclosed in Patent Document 1.
However, when salts of ascorbic acid-2-phosphate-6-higher fatty acid esters such as APPS are formulated as skin preparations for external use, decomposition tends to occur within the preparation, causing problems such as a change in the external appearance. This decomposition occurs mainly due to hydrolysis of the higher fatty acid bonded via an ester linkage to the hydroxyl group at position 6 of the ascorbic acid. As a result of this decomposition, higher fatty acid salts such as sodium palmitate that are insoluble in water are produced, and these appear as a precipitate within the preparation. In the case of a white turbid formulation such as a cream, this precipitate cannot be seen visually, but in a transparent lotion or beauty essence or the like, the precipitate appears as turbidity or sediment that causes a change in the external appearance of the product. Such a problem (caused by turbidity or sediment) tends to significantly occur in accordance with an increase in the length of higher fatty acid portions of ascorbic acid-2-phosphate-6-higher fatty acid esters, because such an increase raises the hydrophobicity of higher fatty acid salts generated by decomposition of the esters.
In response to these types of problems, various methods have been proposed for stabilizing the skin external preparation by adding other components to the ascorbic acid-2-phosphate-6-higher fatty acid ester salt.
For example, Patent Document 2 discloses a method of stabilizing a skin external preparation containing a salt of an ascorbic acid-2-phosphate-6-higher fatty acid ester by adding a water-soluble synthetic polymer compound such as a carboxy vinyl polymer and water to the preparation. It is stated that this method inhibits the decomposition and decrease of the salt of the ascorbic acid-2-phosphate-6-higher fatty acid ester within the skin external preparation, and can therefore suppress the generation of precipitates within the skin external preparation over time.
Patent Document 3 discloses a method of stabilizing a skin external preparation containing a salt of an ascorbic acid-2-phosphate-6-higher fatty acid ester by adding a polyhydric alcohol. It is stated that this method can suppress turbidity and the generation of precipitates within the skin external preparation over time.
Patent Document 4 discloses a method of stabilizing a skin external preparation containing a salt of an ascorbic acid-2-phosphate-6-higher fatty acid ester, wherein a polyglycerol fatty acid ester formed from a polyglycerol having an average polymerization degree of 8 to 12 and an unsaturated fatty acid residue of 14 to 22 carbon atoms, and a polyglycerol mono-fatty acid ester formed from a polyglycerol having an average polymerization degree of 2 to 6 and an unsaturated fatty acid residue of 14 to 22 carbon atoms are used as emulsifiers to form an emulsion having an average emulsion particle size of 1 to 200 nm. It is stated that this method inhibits the decomposition of the salt of the ascorbic acid-2-phosphate-6-higher fatty acid ester within the skin external preparation, thereby improving the storage stability and enabling better retention of a beautiful transparent or semi-transparent external appearance.